The present invention relates to electrodes or electrode blanks for forming tapped holes by electrical discharge machining.
It is known to form tapped holes in bodies made of hard metal, such as hardened tool steels, by an operation known as electrical discharge machining. In this operation, the tapped hole is formed by advancing a cylindrical electrode whose outer surface has a form corresponding to the thread to be tapped toward the metal part so that the electrode and the part are initially separated by a small gap which is filled with an insulating fluid, such as oil or water. The cutting or threading process takes place in a tank and the electrode and part to be machined are connected to the terminals of a DC source via appropriate conductors so that the desired potential exists therebetween.
The electrode is then caused to advance toward the part, while being rotated at a rate correlated with the rate of forward feed to produce a screwing motion and when the gap between the electrode and the part decreases below a given value, an arc is formed between the electrode and the part, causing erosion of the metal directly adjacent the electrode. While the arc is maintained, the electrode is advanced into the part, while continuing to be rotated, thereby creating a threaded bore in the part. The movements of the electrode is imparted by a hydraulic motor attached thereto and the rate of feed produced by the hydraulic motor is controlled by spark feedback through a machine control.
Various machines for carrying out this operation are already available on the market, one such machine being marketed by the Easco-Sparcatron Company of Whitmore Lake, Michigan, under the Trademark Spark-Matic, Model ES-162.
The electrodes can be of graphite, a copper-tungsten alloy, or any other composition known to be usable for such component. One suitable type of graphite is sold under the designation Grade KK-10. Suitable electrodes are marketed by Airco-Speers.
During a tapping operation, it is necessary to flush material being removed from the hole being formed. For this purpose, known electrodes are provided with a flushing bore concentric with the axis of the electrode. Since the electrode is being rotated about its axis, the result is that metal located along the axis of the electrode is not removed and remains, in the form of a core, in the tapped hole at the end of the machining operation. It is necessary to perform a subsequent operation to remove that core.
Moreover, since the core lies within the flushing bore, and thus partially blocks that bore, the flow of flushing liquid is impeded. The liquid must then be supplied at a sufficiently high pressure to offset this partial blockage.